Generally, in an image display device such as a television receiver using a cathode-ray tube (CRT) or the like, a horizontal deflection circuit is provided in order to flow a sawtooth wave current in a horizontal deflection coil.
This horizontal deflection circuit is constructed, for example, by connecting a damper diode and a resonance capacitor parallel between the collector and the emitter of an npn-type horizontal output transistor for creating a switching element, connecting a series circuit of a horizontal deflection coil and an S-curve correction capacitor in parallel between the collector and the emitter of the horizontal output transistor, connecting the collector of the horizontal output transistor to the positive electrode of a direct-current power source through a primary winding of a flyback transformer, and connecting the emitter of the horizontal output transistor to the negative electrode of the direct-current power source.
In this horizontal deflection circuit, in the horizontal scanning period, the horizontal output transistor or the damper diode is in a conductive state, and the horizontal deflection current in the horizontal scanning period is maintained by the series resonance of the S-curve correction capacitor and the horizontal defection coil.
At this time, however, since the circuit elements such as the horizontal deflection coil, the horizontal output transistor, the damper diode and the like consume the electric power, the amplitude of the horizontal deflection current gradually decreases during the horizontal scanning period. Thus the amplitude of the horizontal deflection current at the end of the horizontal scanning period is a smaller value than the amplitude of the horizontal deflection current at the start of the horizontal scanning period.
Moreover, since an internal resistance is present in the horizontal deflection coil, the voltage across both ends of the horizontal deflection coil is decreased by the portion of the product of the horizontal deflection current and this internal resistance. Accordingly, as the horizontal deflection current increases, the changing portion of the current decreases.
Generally, since in the image display device, scanning is carried out horizontally from the left side to the right side facing the screen, when it is observed as the image on the cathode-ray tube, the right side of the image looks contracted, and this is called "right contraction." Instead of "right contraction," it may be also called "left expansion."
To correct this right contraction, in an image display device using the cathode-ray tube, a saturable reactor magnetically biased by a permanent magnet called a horizontal linearity coil is used. In this horizontal linearity coil, the working point of the characteristic curve indicating the inductance value corresponding to the current value is shifted in the current axial direction by the permanent magnet. Accordingly, the horizontal linearity coil has the property in which the inductance is different depending on the current flowing direction.
When this horizontal linearity coil is connected in series to the horizontal deflection coil so that the inductance may be maximum upon the start of horizontal scanning, in the first half of horizontal scanning, apparently, the impedance of the horizontal deflection coil becomes large and the horizontal deflection current is limited thereby, but in the latter half of horizontal scanning, the horizontal deflection current is decreased by power consumption in the horizontal deflection circuit, but since the inductance of the horizontal linearity coil is decreased, apparently, the impedance of the horizontal deflection coil is small, so that decrease of current can be prevented.
In such linearity correction by the horizontal linearity coil, the correction effect thereof is determined by the quality of the magnetic material used in the horizontal linearity coil and the strength of the magnet, the relation between the current and the inductance may change sharply more or less, and the correction effect may change rapidly depending on the linearity changes, and it is hard to obtain an ideal correction effect.
Image distortion in the image display device using the cathode-ray tube includes, aside from the right contraction distortion mentioned above, an image distortion called a pincushion distortion. This pincushion distortion occurs due to a difference in the scanning width on the screen, in the upper and lower portions and central portion of the screen, with respect to the same deflection angle of the electron beam.
To correct this pincushion distortion, it is known to modulate the horizontal deflection current by using a saturable reactor, same as in the case of linearity correction. In this case, since the horizontal deflection current amount varies between the upper and lower portions and the central portion of the screen, the correction amount of pincushion distortion is also different, and an imbalance is caused in the correction state of pincushion between the right and left sides of the screen.
Moreover, in the aforementioned image display device using the cathode-ray tube, if the center of horizontal deflection and the central position of the screen are not matched due to fluctuations in the image pickup tube and the horizontal deflection coil, an independent horizontal centering circuit was needed in order to match them.
Further, since the horizontal linearity coil is used by connecting in series to the horizontal deflection coil, the impedance of this horizontal deflection coil must be set smaller by the corresponding amount, and the amplitude of the required horizontal deflection current becomes larger.
In the light of these problems, it is hence an object of the invention to adjust the correction amount of the horizontal linearity so that an ideal correction effect may be obtained.
It is also an object of the invention to suppress the amplitude of the horizontal deflection current to be small.
It is other object of the invention to get rid of an independent horizontal centering circuit.